Conventionally, an uninterruptible power supply device includes a converter for converting alternating-current (AC) power from an AC power source into direct-current (DC) power, an inverter for converting DC power generated by the converter or DC power of a power storage device into AC power and supplying the AC power to a load, a bypass circuit connected between the AC power source and the load, and a control device for controlling those components.
In the uninterruptible power supply device above, during normal operation in which AC power is supplied normally from the AC power source, AC power generated by the inverter is supplied to the load, whereas when the inverter is failed, AC power from the AC power source is supplied to the load through the bypass circuit. This system is called an on-line power supply system. The on-line power supply system has an advantage that high-quality AC power with small voltage fluctuations can be supplied to the load, irrespective of the quality of the AC power source. Meanwhile, this system has the drawback of low efficiency because power loss always occurs in the inverter.
In another system, AC power from the AC power source is supplied to the load via the bypass circuit during normal operation, whereas during power failure, DC power of the power storage device is converted by the inverter into AC power to supply the load. This system is called a standby power supply system. The standby power supply system has an advantage that power loss is small and the efficiency is high. Meanwhile, this system has the drawback that low-quality AC power with large voltage fluctuations that is supplied from the AC power source is supplied to the load.
For example, Japanese Patent Laying-Open No. 2010-115008 (Patent Document 1) discloses an uninterruptible power supply device configured to detect a state of AC power supplied from an AC power source and select one of the on-line power supply system and the standby power supply system according the detection result.